Various moisture absorbing agents have been proposed as measures for removing moisture in air. However, their properties such as moisture pickups, moisture absorption rates, and moisture absorption/desorption characteristics have both advantages and disadvantages, so that such moisture absorbing agents need to be selected depending on intended uses.
Lithium chloride and calcium chloride, for example, have the advantages of a great moisture pickup and a high moisture absorption rate, but are disadvantageous in that they deliquesce and thus liquefy after absorbing moisture. Moisture absorbing agents, such as silica gel and zeolite, have the advantage of being repeatedly usable because of their moisture adsorption/desorption properties. However, they are disadvantageous in that their moisture pickups are small, they require high temperatures for regeneration, they are apt to crush upon repeated moisture absorption/desorption, and their mixing with resins is difficult. Water absorbing resins typified by polyacrylic acid salts, moreover, are excellent in water absorbing performance, but are not fully satisfactory from the point of view of hygroscopicity.
Of various moisture absorbing agents, the following materials have been proposed as moisture absorbing agents having high moisture absorption properties and high moisture desorption properties and excellent in moisture absorption/desorption rate: One of them is a moisture absorbing/desorbing polymer comprising a vinyl polymer containing 1.0 to 8.0 meq/g of potassium carboxylate groups, and having a crosslinked structure obtained by copolymerization of divinylbenzene (Patent Document 1). The other material is moisture absorbing/desorbing ultrafine particles comprising a crosslinked polymer containing 1.0 to 10.0 meq/g of carboxylate groups and 0.1 to 2.0 meq/g of sulfonic acid groups and/or sulfonate groups as polar groups, the ultrafine particles having an average primary particle diameter of 0.1 μm or less, and saturated moisture absorption coefficients, at 20° C. and 65% RH and 90% RH, of 20% by weight or more and 40% by weight or more, respectively (Patent Document 2).